Numerous systems have been proposed for coupling a magnetically coded signal to a vehicle, such as an automotive vehicle traversing a line of a roadway. These systems are characterized by signposts, each including an array of magnetic fields, generally derived from permanent magnets embedded in a roadway, in combination with a transducer mounted on the vehicle for deriving an electric signal in response to the magnetic field. The magnetic field is distributed along the roadway in a coded manner to enable the transducer to derive a multi-bit signal as it travels down a lane in which the signpost is located. The proposed prior art systems are of two categories, namely: systems responsive to a rate of change of magnetic field in response to relative movement between the vehicle and the signpost, the systems including transducers responsive to the magnitude or polarity of the magnetic field.
The former systems suffer from the inherent disadvantage of requiring movement between the vehicle and the signpost. Thereby, if the vehicle stops or moves very slowly over a signpost, the transducer is unable to derive a signal that accurately reflects the coded signal of the signpost. Also, systems of this type are subject to cumulative errors since a register must always be monitoring rate of change rather than amplitude or polarity variations.
Systems that have been proposed wherein the intensity of a magnetic field, either amplitude or polarity, in contrast to rate of magnetic field change, is coupled from a roadway to a vehicle are found in: U.S. Pat. Nos. 1,803,288; 1,803,289; 1,803,290; 1,803,291; 1,803,292, all of which are issued to Charles Adler, Jr., Lippmann et al, 3,297,866, Stevens et al 3,493,923, as well as U.K. Pat. Nos. 797,056 and 823,149. Of these proposed prior art devices, the most sophisticated and the one most suited for coupling coded signals to an automotive vehicle appears to be disclosed by the Stevens et al patent.
In the patent disclosed by Stevens, a signpost includes a single row of permanent magnets that are disposed along the length of a lane of a roadway. The magnets are polarity coded and vertically positioned so that a magnetic field amplitude or polarity detector array on a vehicle traversing the signpost is alternately responsive to polarity coded, vertically directed magnetic lines of force. The vehicle carried transducer array extends across a substantial portion of the vehicle width, i.e., in a direction transverse to the direction of travel of the vehicle. This result is achieved by providing on each vehicle several transversely spaced field concentrators, each of which includes an air gap in which is located a Hall plate. The field concentrators are mounted on a pair of magnetically permeable flux collector bars that extend substantially across the entire width of the vehicle to establish parallel magnetic field paths between the magnetic fields of the sign post and the field concentrators and Hall plates. Output signals from the several Hall plates are connected together to the input of a single amplifier which drives circuitry for analyzing the magnetic fields and for deriving an indication of the signpost data.
In the system disclosed by Stevens et al, it appears that an output signal of the detector array would be subject to error if the vehicle is not driving exactly straight on the roadway, i.e., the vehicle is skewed. This is because the multiple Hall plates in the Stevens et al arrangement are possibly subjected to relatively strong magnetic fields at differing portions of the road. Hence, there is a possibility that the magnetic field from a signpost could interact with the magnetic field from some other object in the roadway as the vehicle traverses the roadway. The two interacting magnetic fields could nullify each other so that there is an unpredictable variation in the magnetic field coupled to the Hall plates, depending upon the skew angle of the vehicle. Also, because of the parallel low reluctance paths through the several field concentrators to a plurality of Hall plates, the field traversing each Hall plate is reduced. Hence, there is a tendency for the sensitivity of an array of the type disclosed by Stevens et al to be less than the sensitivity of an array including a single field concentrator and a single Hall plate. Of course, it is desirable to employ a magnetic field amplitude or polarity detector employing a single Hall plate and concentrator because of the cost involved in installing several Hall plates and concentrators on many thousands of vehicles which might be involved in a system for coupling signpost information from a roadway to a vehicle.
Another defect in the prior art magnetic field amplitude or polarity systems is that they fail to take into account the inherent drift of a magnetic field amplitude or polarity transducer as a function of ambient temperature and magnetic field. In experiments that we have conducted, it was determined that ambient changes in magnetic field and temperature cause a base line drift frequently of the same order of magnitude as the magnetic field variations associated with a signpost. Conventional automatic gain control systems did not function adequately because of the extremely long term nature of the ambient variations, as well as because of the necessity to deactivate the automatic gain circuit while a signpost is being detected.
There are other problems which prior art workers have apparently failed to appreciate. In particular, the prior art systems are subject to erroneous results because they do not check to assure that a correct number of pulses are coupled to the vehicle magnetic field transducer from each signpost, and because they do not establish criteria for the spatial distance between the leading and trailing edges of each pulse, or between the trailing and leading edges of adjacent pulses, as a function of possible skew. In experiments we have conducted we have found that the presence of magnetic field anamolies along the length of a roadway can appear as a signpost if precautions are not made to distinguish these anomalies from signpost field variations.
The prior art has also apparently failed to consider the consequences of a vehicle stopping over a signpost and then backing up while over the signpost, either to a position within the sign post or outside of the sign post. If precautions are not taken to compensate for errors due to stopping and backing up over a signpost, erroneous results will occur.
It is, accordingly, an object of the present invention to provide a new and improved apparatus for coupling magnetically coded information from a signpost to a vehicle.
Another object of the invention is to provide a new and improved apparatus for coupling magnetically coded polarity information from a magnetic signpost to a vehicle transversing the signpost.
A further object of the invention is to provide a new and improved magnetic signpost and vehicular detector arrangement that is relatively inexpensive, highly sensitive, and is capable of accurately detecting signpost information even if the vehicle is skewed with respect to a lane including a signpost.
An additional object of the invention is to provide a new and improved apparatus for magnetically coupling signpost signals to a vehicle wherein long term ambient conditions, such as temperature and magnetic field, that would affect the output of a magnetic field polarity sensor are compensated.
A further object of the invention is to provide a new and improved magnetic field amplitude signpost for coupling magnetically polarized data to a vehicle including a magnetic field polarity transducer.
A further object of the invention is to provide a new and improved system for coupling magnetic field signpost information to a vehicle wherein processing circuitry is provided to minimize or eliminate errors due to magnetic field anomalies along a roadway being traversed by the vehicle.
Another object of the invention is to provide a new and improved system for coupling magnetic field polarity information from a signpost to a vehicle wherein pulses detected by a magnetic field polarity detector on the vehicle are processed as a function of width between leading and trailing edges within the pulse and between adjacent pulses, as a function of distance traveled by the vehicle, to substantially prevent magnetic anomalies in the roadway from being indicated as a signpost.
Yet another object of the invention is to provide a new and improved system for coupling coded signals from a magnetic signpost to a vehicle wherein accurate signpost information is derived even though the vehicle stops and backs up over a signpost.